The endothelium forms the critical barrier restricting the movement of drugs and endogenous molecules from the circulating blood to the underlying tissue cells. Little molecular information is available about the luminal endothelial cell surface. The applicant s laboratory has: i) developed a new technique for purifying this cell surface directly from tissue; ii) elucidated basic selective transport mechanisms that move proteins into and across endothelium; and iii) purified specialized vesicles in endothelium called caveolae that contain the molecular machinery necessary for this transport. With this knowledge, site-directed delivery systems will be created in order to confer accessibility to the tissue cells by overcoming the endothelial barrier in an organ-specific manner. Endothelial membranes will be purified from many tissues to create a protein map that identifies tissue- and caveolae-specific targets. Monoclonal antibodies achieved in vivo tissue-specific targeting by intravenous injection. By design, the antibodies target the endothelial caveolae in one rat tissue and thereby rapidly accumulate in the intended target organ. The applicant proposes to study the molecular topography of the endothelial cell surface in vivo as a means of creating tissue-specific immunotargeting agents for the endothelium and its caveolae. He will test the in vivo targeting of our antibodies and determine how the endothelium transports them. The applicant also aims to purify and sequence the key targeting proteins. It appears that the quest to acquire basic knowledge at the molecular and cellular level about the endothelium and its caveolae in transport, the applicant has discovered a direct application towards the creation of tissue-specific targeting agents potentially useful in drug and gene therapy of many diseases.